1. Field of the Invention
This invention relates to improved low temperature synthetic hydraulic fluids comprised of a 2 centistoke hydrogenated alpha-olefin oligomer, an ester or mixture of esters, and a viscosity index improver, said ester(s) and viscosity index improver selected to impart specific characteristics to the fluid which may also contain other conventional additives.
2. Description of the Prior Art
Military specifications for petroleum based hydraulic fluids designed for use in the -65.degree. F. to 160.degree. F. temperature range are defined by MIL-H-5606. These fluids give the military acceptable low temperature operating capabilities, however, due to their high volatility and relatively low flash point (179.6.degree. F. minimum) these fluids present a substantial fire hazard and therefore are not suitable for use in many applications.
As a result of the flammability problems associated with the MIL-H-5606 fluids, more fire-resistant hydraulic fluids have been developed and are specified in MIL-H-83282. MIL-H-83282 fluids are based on 4 centistoke synthetic hydrocarbon base stocks and have significantly higher flash points (400.degree. F. minimum). This is not, however, accomplished without some sacrifice of low temperature properties. MIL-H-83282B fluids are recommended for use only to -40.degree. F.
It would be highly desirable if hydraulic fluids having the -65.degree. F. capabilities of the MIL-H-5606 fluids and exhibiting the substantially improved fire resistance of MIL-H-83282 fluids were available. It would be even more advantageous if such fluids were based on readily available and economical 2 centistoke synthetic hydrocarbon basestock obtained from an alpha-olefin oligomerization process and if the products possessed all of the other properties necessary for commercial hydraulic fluids.
The low molecular weight fraction obtained from alpha-olefin oligomerization processes, which is comprised primarily of dimer and has a 210.degree. F. viscosity of about 2 centistokes, is generally considered to be unsuitable for most lubrication applications. Therefore, in most oligomerization processes the oligomeric product is fractionated to remove any unreacted olefin and the olefin dimers (see for example U.S. Pat. Nos. 3,907,924, 4,175,046 and 4,282,392). The fraction(s) which contain predominantly trimer and/or tetramer oligomers and which typically have 210.degree. F. viscosities in the 4 to 6 centistoke range are then utilized for commercial and industrial uses.
It is known to blend certain esters with the higher oligomers of alpha-olefins (trimer and tetramer) to obtain useful synthetic oils. For example, in U.S. Pat. No. 4,175,046 to Coant et al. compositions useful as lubricants for marine and industrial diesel engines and comprised of a mixture of esters and a hydrogenated alpha-olefin oligomer are described. The hydrogenated oligomers employed by Coant et al. typically have 210.degree. F. viscosities of about 6 centistokes and the use of viscosity index improvers is expressly prohibited. Two centistoke alpha-olefin oligomers would not be acceptable for use in the formulations of U.S. Pat. No. 4,175,046 since products totally unsuitable for use as engine oils would be obtained.
In view of the limited number of applications heretofore developed for the 2 centistoke fraction, efforts to recycle olefin dimers in the oligomerization process to further react the dimer with additional alpha-olefin have been made but have not been entirely successful. This is due primarily to the significantly reduced reactivity of the dimer. It has thus been suggested for some commercial operations that the most economical means of disposing of the 2 centistoke fraction is to burn the product for its fuel value. It would be highly desirable if useful lubricant compositions based on 2 centistoke alpha-olefin oligomers were available.